Evaluation of GafChromic EBT prototype B for external beam dose verification

The capability of the new GafChromic EBT prototype B for external beam dose verification is investigated in this paper. First the general characteristics of this film (dose response, postirradiation coloration, influence of calibration field size) were derived using a flat-bed scanner. In the dose range from 0.1 to 8 Gy, the sensitivity of the EBT prototype B film is ten times higher than the response of the GafChromic HS, which so far was the GafChromic film with the highest sensitivity. Compared with the Kodak EDR2 film, the response of the EBT is higher by a factor of 3 in the dose range from 0.1 to 8 Gy. The GafChromic EBT almost does not show a temporal growth of the optical density and there is no influence of the chosen calibration field size on the dose response curve obtained from this data. A MatLab program was written to evaluate the two-dimensional dose distributions from treatment planning systems and GafChromic EBT film measurements. Verification of external beam therapy (SRT, IMRT) using the above-mentioned approach resulted in very small differences between the planned and the applied dose. The GafChromic EBT prototype B together with the flat-bed scanner and MatLab is a successful approach for making the advantages of the GafChromic films applicable for verification of external beam therapy.     

Accurate dosimetry with GafChromic EBT film of a 6 MV photon beam in water: what level is achievable?

This paper focuses on the accuracy, in absolute dose measurements, with GafChromicTM EBT film achievable in water for a 6 MV photon beam up to a dose of 2.3 Gy. Motivation is to get an absolute dose detection system to measure up dose distributions in a (water) phantom, to check dose calculations. An Epson 1680 color (red green blue) transmission flatbed scanner has been used as film scanning system, where the response in the red color channel has been extracted and used for the analyses. The influence of the flatbed film scanner on the film based dose detection process was investigated. The scan procedure has been optimized; i.e. for instance a lateral correction curve was derived to correct the scan value, up to 10%, as a function of optical density and lateral position. Sensitometric curves of different film batches were evaluated in portrait and landscape scan mode. Between various batches important variations in sensitometric curve were observed. Energy dependence of the film is negligible, while a slight variation in dose response is observed for very large angles between film surface and incident photon beam. Improved accuracy in absolute dose detection can be obtained by repetition of a film measurement to tackle at least the inherent presence of film inhomogeneous construction. We state that the overall uncertainty is random in absolute EBT film dose detection and of the order of 1.3% (1 SD) under the condition that the film is scanned in a limited centered area on the scanner and at least two films have been applied. At last we advise to check a new film batch on its characteristics compared to available information, before using that batch for absolute dose measurements.     

EBT3胶片数字化的横向响应伪影消除方法

目的：消除EBT3胶片数字化过程中的横向响应伪影,优化EBT3胶片的治疗计划验证结果。方法：覆盖双面镀膜的减反射玻璃进行胶片数字化,通过净光密度与扫描仪不同位置间的抛物线拟合关系来消除横向响应伪影后,借助剂量刻度曲线将胶片净光密度转换为剂量。使用辐照面积较大的治疗计划进行验证,对胶片与计划剂量分布进行γ分析。结果：在3%/3 mm标准下对不小于0.1 Gy的剂量点进行γ分析,消除横向响应伪影后胶片与计划剂量分布的γ通过率为95%,相比未消除横向响应伪影的剂量分布提升了3%的通过率。结论：利用该方法可以有效消除横向响应伪影,提高EBT3胶片治疗计划验证的γ通过率。     

GafChromic EBT film dosimetry with flatbed CCD scanner: a novel background correction method and full dose uncertainty analysis

Film dosimetry using radiochromic EBT film in combination with a flatbed charge coupled device scanner is a useful method both for two-dimensional verification of intensity-modulated radiation treatment plans and for general quality assurance of treatment planning systems and linear accelerators. Unfortunately, the response over the scanner area is nonuniform, and when not corrected for, this results in a systematic error in the measured dose which is both dose and position dependent. In this study a novel method for background correction is presented. The method is based on the subtraction of a correction matrix, a matrix that is based on scans of films that are irradiated to nine dose levels in the range 0.08-2.93 Gy. Because the response of the film is dependent on the film's orientation with respect to the scanner, correction matrices for both landscape oriented and portrait oriented scans were made. In addition to the background correction method, a full dose uncertainty analysis of the film dosimetry procedure was performed. This analysis takes into account the fit uncertainty of the calibration curve, the variation in response for different film sheets, the nonuniformity after background correction, and the noise in the scanned films. The film analysis was performed for film pieces of size 16 x 16 cm, all with the same lot number, and all irradiations were done perpendicular onto the films. The results show that the 2-sigma dose uncertainty at 2 Gy is about 5% and 3.5% for landscape and portrait scans, respectively. The uncertainty gradually increases as the dose decreases, but at 1 Gy the 2-sigma dose uncertainty is still as good as 6% and 4% for landscape and portrait scans, respectively. The study shows that film dosimetry using GafChromic EBT film, an Epson Expression 1680 Professional scanner and a dedicated background correction technique gives precise and accurate results. For the purpose of dosimetric verification, the calculated dose distribution can be compared with the film-measured dose distribution using a dose constraint of 4% (relative to the measured dose) for doses between 1 and 3 Gy. At lower doses, the dose constraint must be relaxed.     

Precautions and strategies in using a commercial flatbed scanner for radiochromic film dosimetry

The purpose of this study was to investigate the value of a commercially available flatbed scanner for film dosimetry with radiochromic film for external radiotherapy. The EPSON Pro 1680 Expression scanner was examined as a densitometer for two-dimensional film dosimetry with Gafchromic EBT film. An accurate and efficient scanning procedure was established. Possible drift and warm-up effects of the scanner were studied and the direct physical influence of the scanner light on the radiochromic film was assessed. Next, we investigated the scan field uniformity. Also, we examined if the accuracy of radiochromic film was improved by subtracting the optical density of the unirradiated blank film from the optical density of the irradiated film. To assess the accuracy of Gafchromic EBT film when the EPSON scanner was used as a densitometer, the depth dose of a 2 x 15 cm(2) field and the in-plane and cross-plane profiles of a 15 x 15 cm(2) field were measured and compared with diamond detector measurements. When taking consecutive scans, we found that the optical density taken from the first scan was about 1% higher than the optical density taken from subsequent scans. We attribute this to the warming up of the lamp of the scanner. Longer-term drift of the scanner was found to be absent. We found that the use of a correction matrix was necessary to correct for the non-uniform scanner response over the scan field. Subtracting the optical density of the unirradiated blank film from the irradiated film improves the precision of the Gafchromic EBT film. Depth dose and profile measurements with Gafchromic EBT film and the diamond detector are in agreement within 2.5%. The EPSON Pro 1680 Expression scanner is an excellent tool for accurate two-dimensional film dosimetry with Gafchromic EBT film provided that some precautions and corrections are taken into account.     

Clinical use of EBT model Gafchromic film in radiotherapy

The Gafchromic EBT was recently introduced in film dosimetry for external beam therapy (EBT). The high spatial resolution, weak energy dependence, and near-tissue equivalence of EBT films make them suitable for measurement of dose distributions in radiotherapy, especially intensity-modulated radiation therapy (IMRT). Starting with a sensitometric curve and dose uncertainty relative to the flatbed scanner, the goal of this study was to find an efficient method of correcting for light scattering, and to compare dose distribution supplied by Gafchromic EBT with the distribution obtained with a 2D ion-chamber detector system. Light scattering was analyzed for different levels of dose, and was found to depend on the red-scale value as well as the position of the pixel on the scanner. Many "uniform" films were exposed at different levels of dose to create a two-dimensional matrix correction to take this effect into account. The dose distribution obtained for three clinical beams (10 x 10, 15 x 15 cm open fields and 12 x 12 cm wedge 60 degrees field) were in agreement with those supplied by the 2D array. Gamma index <1 (using 5 mm distance and 5% dose as constraints) for the three fields considered was reached in an average of 98% of the points.     

Comparison of the epson expression 1680 flatbed and the vidar VXR-16 dosimetry PRO film scanners for use in IMRT dosimetry using gafchromic and radiographic film

Intensity-modulated radiotherapy (IMRT) treatment plan verification is often done using Kodak EDR2 film and a Vidar Dosimetry PRO film digitizer. However, since many hospitals are moving towards a filmless environment, access to a film processor may not be available. Therefore, we have investigated a newly available Gafchromic EBT film for IMRT dosimetry. Planar IMRT dose distributions are delivered to both EBT and EDR2 film and scanned with the Vidar VXR-16 as well as an Epson Expression 1680 flatbed scanner. The measured dose distributions are then compared to those calculated with a Pinnacle treatment planning system. The IMRT treatments consisted of 7-9 6 MV beams for treatment of prostate, head and neck, and a few other sites. The films were analyzed using FilmQATM (3cognition LLC) software. Comparisons between measured and calculated dose distributions are reported as dose difference (DD) (pixels within +/-5%), distance to agreement (DTA) (3 mm), as well as gamma values (y) (dose= +/-3%, dist. =2 mm). Using EDR2 with the Vidar scanner is an established technique and agreement between calculated and measured dose distributions was better than 90% in all indices (DD, DTA, and gamma). However, agreement with calculations deteriorated reaching the lower 80% for EBT film scans with the Vidar scanner in logarithmic mode. The EBT Vidar scans obtained in linear mode showed an improved agreement to the upper 80% range, but artifacts were still observed across the scan. These artifacts were very distinct in all EBT scans and can be attributed to the way the film is transported through the scanner. In the Epson scanner both films are rigidly immobilized and the light source scans over the film. It was found that the Epson scanner performed equally well with both types of film giving agreement to better than 90% in all indices.     

Characterization and use of EBT radiochromic film for IMRT dose verification

We present an evaluation of a new and improved radiochromic film, type EBT, for its implementation to IMRT dose verification. Using a characterized flat bed color CCD scanner, the film's dose sensitivity, uniformity, and speed of development post exposure were shown to be superior to previous types of radiochromic films. The film's dose response was found to be very similar to ion chamber scans in water through comparisons of depth dose and lateral dose profiles. The effect of EBT film polarization with delivered dose and film scan orientation was shown to have a significant effect on the scanner's OD readout. In addition, the film's large size, flexibility, and the ability to submerge it in water for relatively short periods of time allowed for its use in both water and solid water phantoms to verify TomoTherapy IMRT dose distributions in flat and curved dose planes. Dose verification in 2D was performed on ten IMRT plans (five head and neck and five prostate) by comparing measured EBT dose distributions to TomoTherapy treatment planning system calculated dose. The quality of agreement was quantified by the gamma index for four sets of dose difference and distance to agreement criteria. Based on this study, we show that EBT film has several favorable features that allow for its use in routine IMRT patient-specific QA.     

Dosimetric characterization of GafChromic EBT film and its implication on film dosimetry quality assurance

The suitability of radiochromic EBT film was studied for high-precision clinical quality assurance (QA) by identifying the dose response for a wide range of irradiation parameters typically modified in highly-conformal treatment techniques. In addition, uncertainties associated with varying irradiation conditions were determined. EBT can be used for dose assessment of absorbed dose levels as well as relative dosimetry when compared to absolute absorbed dose calibrated using ionization chamber results. For comparison, a silver halide film (Kodak EDR-2) representing the current standard in film dosimetry was included. As an initial step a measurement protocol yielding accurate and precise results was established for a flatbed transparency scanner (Epson Expression 1680 Pro) that was utilized as a film reading instrument. The light transmission measured by the scanner was found to depend on the position of the film on the scanner plate. For three film pieces irradiated with doses of 0 Gy, approximately 1 Gy and approximately 7 Gy, the pixel values measured in portrait or landscape mode differed by 4.7%, 6.2% and 10.0%, respectively. A study of 200 film pieces revealed an excellent sheet-to-sheet uniformity. On a long time scale, the optical development of irradiated EBT film consisted of a slow but steady increase of absorbance which was not observed to cease during 4 months. Sensitometric curves of EBT films obtained under reference conditions (SSD = 95 cm, FS = 5 x 5 cm(2), d = 5 cm) for 6, 10 and 25 MV photon beams did not show any energy dependence. The average separation between all curves was only 0.7%. The variation of the depth d (range 2-25 cm) in the phantom did not affect the dose response of EBT film. Also the influence of the radiation field size (range 3 x 3-40 x 40 cm(2)) on the sensitometric curve was not significant. For EDR-2 films maximum differences between the calibration curves reached 7-8% for X6MV and X25MV. Radiochromic EBT film, in combination with a flatbed scanner, presents a versatile system for high-precision dosimetry in two dimensions, provided that the intrinsic behaviour of the film reading device is taken into account. EBT film itself presents substantial improvements on formerly available models of radiographic and a radiochromic film and its dosimetric characteristics allow us to measure absorbed dose levels in a large variety of situations with a single calibration curve.     

Dose and energy dependence of response of Gafchromic XR-QA film for kilovoltage x-ray beams

There is a growing interest in Gafchromic films for patient dosimetry in radiotherapy and in radiology. A new model (XR-QA) with high sensitivity to low dose was tested in this study. The response of the film to different x-ray beam energies (range 28-145 kVp with various filtrations, dose range 0-100 mGy) and to visible light was investigated, together with the after exposure darkening properties. Exposed films were digitized with a commercially available, optical flatbed scanner. A single functional form for dose versus net pixel value variation has been determined for all the obtained calibration curves, with a unique fit parameter different for each of the used x-ray beams. The film response was dependent on beam energy, with higher colour variations for the beams in the range 80-140 kVp. Different sources of uncertainties in dose measurements, governed by the digitalization process, the film response uniformity and the calibration curve fit procedure, have been considered. The overall one-sigma dose measurement uncertainty depended on the beam energy and decreased with increasing absorbed dose. For doses above 10 mGy and beam energies in the range 80-140 kVp the total uncertainty was less than 5%, whereas for the 28 kVp beam the total uncertainty at 10 mGy was about 10%. The post-exposure colour variation was not negligible in the first 24 h after the exposure, with a consequent increase in the calculated dose of about 10%. Results of the analysis of the sensitivity to visible light indicated that a short exposure of this film to ambient and scanner light during the measurements will not have a significant impact on the radiation dosimetry.     

Precise radiochromic film dosimetry using a flat-bed document scanner

In this study, a measurement protocol is presented that improves the precision of dose measurements using a flat-bed document scanner in conjunction with two new GafChromic film models, HS and Prototype A EBT exposed to 6 MV photon beams. We established two sources of uncertainties in dose measurements, governed by measurement and calibration curve fit parameters contributions. We have quantitatively assessed the influence of different steps in the protocol on the overall dose measurement uncertainty. Applying the protocol described in this paper on the Agfa Arcus II flat-bed document scanner, the overall one-sigma dose measurement uncertainty for an uniform field amounts to 2% or less for doses above around 0.4 Gy in the case of the EBT (Prototype A), and for doses above 5 Gy in the case of the HS model GafChromic film using a region of interest 2 X 2 mm2 in size.     

Important considerations for radiochromic film dosimetry with flatbed CCD scanners and EBT GAFCHROMIC film

In this study, we present three significant artifacts that have the potential to negatively impact the accuracy and precision of film dosimetry measurements made using GAFCHROMIC EBT radiochromic film when read out with CCD flatbed scanners. Films were scanned using three commonly employed instruments: a Macbeth TD932 spot densitometer, an Epson Expression 1680 CCD array scanner, and a Microtek ScanMaker i900 CCD array scanner. For the two scanners we assessed the variation in optical density (OD) of GAFCHROMIC EBT film with scanning bed position, angular rotation of the film with respect to the scan line direction, and temperature inside the scanner due to repeated scanning. Scanning uniform radiochromic films demonstrated a distinct bowing effect in profiles in the direction of the CCD array with a nonuniformity of up to 17%. Profiles along a direction orthogonal to the CCD array demonstrated a 7% variation. A strong angular dependence was found in measurements made with the flatbed scanners; the effect could not be reproduced with the spot densitometer. An IMRT quality assurance film was scanned twice rotating the film 90' between the scans. For films scanned on the Epson scanner, up to 12% variation was observed in unirradiated EBT films rotated between 0 degrees and 90 degrees, which decreased to approximately 8% for EBT films irradiated to 300 cGy. Variations of up to 80% were observed for films scanned with the Microtek scanner. The scanners were found to significantly increase the film temperature with repeated scanning. Film temperature between 18 and 33 degrees C caused OD changes of approximately 7%. Considering these effects, we recommend adherence to a strict scanning protocol that includes: maintaining the orientation of films scanned on flatbed scanners, limiting scanning to the central portion of the scanner bed, and limiting the number of consecutive scans to minimize changes in OD caused by film heating.     

Accurate skin dose measurements using radiochromic film in clinical applications

Megavoltage x-ray beams exhibit the well-known phenomena of dose buildup within the first few millimeters of the incident phantom surface, or the skin. Results of the surface dose measurements, however, depend vastly on the measurement technique employed. Our goal in this study was to determine a correction procedure in order to obtain an accurate skin dose estimate at the clinically relevant depth based on radiochromic film measurements. To illustrate this correction, we have used as a reference point a depth of 70 micron. We used the new GAFCHROMIC dosimetry films (HS, XR-T, and EBT) that have effective points of measurement at depths slightly larger than 70 micron. In addition to films, we also used an Attix parallel-plate chamber and a home-built extrapolation chamber to cover tissue-equivalent depths in the range from 4 micron to 1 mm of water-equivalent depth. Our measurements suggest that within the first millimeter of the skin region, the PDD for a 6 MV photon beam and field size of 10 x 10 cm2 increases from 14% to 43%. For the three GAFCHROMIC dosimetry film models, the 6 MV beam entrance skin dose measurement corrections due to their effective point of measurement are as follows: 15% for the EBT, 15% for the HS, and 16% for the XR-T model GAFCHROMIC films. The correction factors for the exit skin dose due to the build-down region are negligible. There is a small field size dependence for the entrance skin dose correction factor when using the EBT GAFCHROMIC film model. Finally, a procedure that uses EBT model GAFCHROMIC film for an accurate measurement of the skin dose in a parallel-opposed pair 6 MV photon beam arrangement is described.     

Optical imaging analysis of microscopic radiation dose gradients in Gafchromic EBT film using a digital microscope

By providing superior localization and immobilization, stereotactic radiosurgery (SRS) is capable of delivering millimeter spheres of dose to intracranial targets with submillimeter precision. Several authors have proposed new SRS solutions to dramatically reduce beam penumbra to hundreds of microns. These solutions require new quality assurance methods capable of penumbra measurement at the micron scale. This article examines the capability of a digital microscope, with translation stage and associated software, to resolve dose gradients in Gafchromic EBT film at this level. To produce very steep penumbra, films were irradiated in phantom beneath pinhole collimators using lower energy x rays (100 kVp, 300 kVp, and Iridium-192) and minimal geometric penumbra contribution. For film analysis, a method was developed which improved the signal to noise ratio by finding the center of the irradiation spot, generating several radial dose profiles and averaging these to obtain the final off-axis dose profile. Optical density was converted to dose using a calibration curve. The experimentally determined off-axis dose profiles were compared with MCNP Monte Carlo simulations which replicated the irradiation geometry and served to validate our measured data. The measured 80%-20% penumbral widths were 46 microm +/- 26 microm (100 kVp, 2 mm field size), 69 microm +/- m 27 microm (300 kVp, 2 mm field size), and 241 microm +/-31 microm (Ir-192, 1 mm field size). These penumbral widths agreed with Monte Carlo simulations within experimental uncertainty. Our findings suggest that reading Gafchromic EBT films using a digital microscope with translation stage is suitable for the quality assurance of very sharp penumbra able to resolve gradients to within at least 30 microm.     

Evaluation of a novel triple-channel radiochromic film analysis procedure using EBT2

A novel approach to read out radiochromic film was introduced recently by the manufacturer of GafChromic film. In this study, the performance of this triple-channel film dosimetry method was compared against the conventional single-red-channel film dosimetry procedure, with and without inclusion of a pre-irradiation (pre-IR) film scan, using EBT2 film and kilo- and megavoltage photon beams up to 10 Gy. When considering regions of interest averaged doses, the triple-channel method and both single-channel methods produced equivalent results. Absolute dose discrepancies between the triple-channel method, both single-channel methods and the treatment planning system calculated dose values, were no larger than 5 cGy for dose levels up to 2.2 Gy. Signal to noise in triple-channel dose images was found to be similar to signal to noise in single-channel dose images. The accuracy of resulting dose images from the triple- and single-channel methods with inclusion of pre-IR film scan was found to be similar. Results of a comparison of EBT2 data from a kilovoltage depth dose experiment to corresponding Monte Carlo depth dose data produced dose discrepancies of 9.5 ± 12 cGy and 7.6 ± 6 cGy for the single-channel method with inclusion of a pre-IR film scan and the triple-channel method, respectively. EBT2 showed to be energy sensitive at low kilovoltage energies with response differences of 11.9% and 15.6% in the red channel at 2 Gy between 50-225 kVp and 80-225 kVp photon spectra, respectively. We observed that the triple-channel method resulted in non-uniformity corrections of ±1% and consistency values of 0-3 cGy for the batches and dose levels studied. Results of this study indicate that the triple-channel radiochromic film read-out method performs at least as well as the single-channel method with inclusion of a pre-IR film scan, reduces film non-uniformity and saves time with elimination of a pre-IR film scan.     

Radiochromic film dosimetry with flatbed scanners: a fast and accurate method for dose calibration and uniformity correction with single film exposure

Film dosimetry is an attractive tool for dose distribution verification in intensity modulated radiotherapy (IMRT). A critical aspect of radiochromic film dosimetry is the scanner used for the readout of the film: the output needs to be calibrated in dose response and corrected for pixel value and spatial dependent nonuniformity caused by light scattering; these procedures can take a long time. A method for a fast and accurate calibration and uniformity correction for radiochromic film dosimetry is presented: a single film exposure is used to do both calibration and correction. Gafchromic EBT films were read with two flatbed charge coupled device scanners (Epson V750 and 1680Pro). The accuracy of the method is investigated with specific dose patterns and an IMRT beam. The comparisons with a two-dimensional array of ionization chambers using a 18 x 18 cm2 open field and an inverse pyramid dose pattern show an increment in the percentage of points which pass the gamma analysis (tolerance parameters of 3% and 3 mm), passing from 55% and 64% for the 1680Pro and V750 scanners, respectively, to 94% for both scanners for the 18 x 18 open field, and from 76% and 75% to 91% for the inverse pyramid pattern. Application to an IMRT beam also shows better gamma index results, passing from 88% and 86% for the two scanners, respectively, to 94% for both. The number of points and dose range considered for correction and calibration appears to be appropriate for use in IMRT verification. The method showed to be fast and to correct properly the nonuniformity and has been adopted for routine clinical IMRT dose verification.     

The use of new GAFCHROMIC EBT film for 125I seed dosimetry in Solid Water phantom

Radiochromic film dosimetry has been extensively used for intravascular brachytherapy applications for near field within 1 cm from the sources. With the recent introduction of new model of radiochromic films, GAFCHROMIC EBT, with higher sensitivity than earlier models, it is promising to extend the distances out to 5 cm for low dose rate (LDR) source dosimetry. In this study, the use of new model GAFCHROMIC EBT film for 125I seed dosimetry in Solid Water was evaluated for radial distances from 0.06 cm out to 5 cm. A multiple film technique was employed for four 125I seeds (Implant Sciences model 3500) with NIST traceable air kerma strengths. Each experimental film was positioned in contact with a 125I seed in a Solid Water phantom. The products of the air kerma strength and exposure time ranged from 8 to 3158 U-h, with the initial air kerma strength of 6 U in a series of 25 experiments. A set of 25 calibration films each was sequentially exposed to one 125I seed at about 0.58 cm distance for doses from 0.1 to 33 Gy. A CCD camera based microdensitometer, with interchangeable green (520 nm) and red (665 nm) light boxes, was used to scan all the films with 0.2 mm pixel resolution. The dose to each 125I calibration film center was calculated using the air kerma strength of the seed (incorporating decay), exposure time, distance from seed center to film center, and TG43U1S1 recommended dosimetric parameters. Based on the established calibration curve, dose conversion from net optical density was achieved for each light source. The dose rate constant was determined as 0.991 cGy U(-1)h(-1) (+/-6.9%) and 1.014 cGy U(-1)h(-1) (+/-6.8%) from films scanned using green and red light sources, respectively. The difference between these two values was within the uncertainty of the measurement. Radial dose function and 2D anisotropy function were also determined. The results obtained using the two light sources corroborated each other. We found good agreement with the TG43U1S1 recommended values of radial dose function and 2D anisotropy function, to within the uncertainty of the measurement. We also verified the dosimetric parameters in the near field calculated by Rivard using Monte Carlo method. The radial dose function values in Solid Water were lower than those in water recommended by TG43U1S1, by about 2%, 3%, 7%, and 14% at 2, 3, 4, and 5 cm, respectively, partially due to the difference in the phantom material composition. Radiochromic film dosimetry using GAFCHROMIC EBT model is feasible in determining 2D dose distributions around low dose rate 125I seed. It is a viable alternative to TLD dosimetry for 125I seed dose characterization.     

Dose area product evaluations with Gafchromic XR-R films and a flat-bed scanner

Gafchromic XR-R films are a useful tool to evaluate entrance skin dose in interventional radiology. Another dosimetric quantity of interest in diagnostic and interventional radiology is the dose area product (DAP). In this study, a method to evaluate DAP using Gafchromic XR-R films and a flat-bed scanner was developed and tested. Film samples were exposed to an x-ray beam of 80 kVp over a dose range of 0-10 Gy. DAP measurements with films were obtained from the digitalization of a film sample positioned over the x-ray beam window during the exposure. DAP values obtained with this method were compared for 23 cardiological interventional procedures with DAP values displayed by the equipment. The overall one-sigma dose measurement uncertainty depended on the absorbed dose, with values below 6% for doses above 1 Gy. A maximum discrepancy of 16% was found, which is of the order of the differences in the DAP measurements that may occur with different calibration procedures. Based on the results presented, after an accurate calibration procedure and a thorough inspection of the relationship between the actual dose and the direct measured quantity (net optical density or net pixel value variation), Gafchromic XR-R films can be used to assess the DAP.     

Gafchromic EBT3胶片在螺旋断层放射治疗计划验证中的应用

目的:评估基于Gafchromic EBT3胶片的剂量测量系统用于螺旋断层放射治疗计划验证的可靠性,确定该系统的正确使用方法。 方法:使用Gafchromic EBT3胶片和Vidar DosimetryPro Advantage Red扫描仪组成的剂量测量系统,测试并确定系统的一些重要特性对测量结果的影响。此外,使用该系统验证螺旋断层放射治疗计划,借助Gamma指数分析对胶片测量的剂量分布与计划系统计算结果之间进行比较。 结果:胶片辐照后一开始透光度随时间变化比较明显,直到约4 h以后胶片着色渐趋饱和,4 h内扫描值变化最高达11.6%。扫描仪扫描重复性相对标准差小于0.5%。胶片正反方向放置扫描结果之间差别小于0.6%、横向和纵向放置扫描结果之间差别最高达7.0%。Gamma参数设置为3%/3 mm时,横向和纵向放置扫描验证平均通过率分别为96.5%±2.9%和95.7%±3.6%,方差分析显示两种扫描方式的验证通过率在a=0.05水平上没有统计学差异。 结论:使用文中的胶片剂量测量系统时,应居中放置扫描,并使验证胶片和刻度胶片保持相同的扫描方向。通过直接建立扫描值与胶片吸收剂量之间的一一对应关系,将验证胶片的扫描值转换成吸收剂量的方法简便易行。     

利用胶片剂量拟合测量呼吸门控时间延迟

【摘要】目的:通过对胶片剂量曲线的拟合来测量呼吸门控时间延迟,从而为临床治疗提供指导。方法:在呼吸门控治疗中使用模体带动胶片做周期运动,并提取相应的剂量曲线。然后使用Matlab软件模拟脉冲出束过程,根据每次脉冲出束形成的剂量曲线在胶片上的位置分布,计算出各像素点的累积剂量,从而得到在一个呼吸周期内形成的剂量曲线。比较不同呼吸门控延迟时间下的剂量曲线与胶片的实际剂量曲线,使用最小二乘法来寻找最优解,得到相应的门控时间延迟。结果:拟合胶片剂量曲线与实际剂量曲线有较好的一致性,开始出束时的门控延迟时间约为（252±20） ms,停止出束时的门控延迟时间约为（10±7） ms,开始出束时的门控延迟时间明显大于停止出束时的延迟时间。结论:基于胶片剂量拟合的方法可以精确地测量呼吸门控的时间延迟。对于任何门控设备,在临床使用前均需测试其门控延迟时间。